Thermostat



April 14, 1959 D. W. LAVIANA THERMOSTAT Filed May 9, 1957 a m r mm, M W m a M 5 W 5 Q w 0 V o I a m E D 8 2 H B a (a, I NUQQR 3 0 uh w w 4 II. M. M H u 0 m 3. W n H H w 11 v m WUQQK WUQQIK HIS ATTORNEY United States Patent Office THERMOSTAT Donald W. Laviana, Pittsford, N.Y., assignor to General Motors Corporation, Delaware Detroit, Mich., a corporation of This invention relates to thermostats, and particularly to snap acting thermostats. Heretofore, snap acting bimetallic thermostats have been designed comprising a prestressed three-legged strip, the outer legs of which are placed in compression by crimping the middle leg. However, prior bimetallic strips of this type have a serious defect in that when the rate of change of temperature is small, the contact pressure is substantially zero at the time of contact separation, thereby causing erratic operation and radio interference. The present invention pertains to a bimetallic strip which is constructed so as to maintain a positive contact pressure until the strip snaps from one position to the other. Accordingly, among my objects are the provision of a snap acting bimetallic thermostat having means for maintaining a predetermined minimum contact pressure irrespective of the rate of temperature change; the further provision of a prestressed bimetallic strip having a pair of compression legs with different stresses and thermal characteristics; and the still further provision of a snap acting bimetallic thermostat having a pair of compression legs and a tension leg including means for establishing different effective lengths of said compression legs.

The aforementioned and other objects are accomplished in the present invention by designing a threelegged bimetallic thermostat so that there is a predetermined force relationship between the two compression legs. Specifically, the bimetallic thermostat of the present invention includes a supporting structure having threaded openings which receive a calibrating screw and a differential adjusting screw, respectively. The bimetallic element comprises a strip, or reed, having a pair of longitudinally extending closed slots therein so as to form a pair of exterior, or marginal legs and a central leg. The central leg is crimped so as to place it in tension and simultaneously place the marginal legs in compression. One end of the reed is rigidly secured to the supporting structure, and the other end of the reed has a contact attached thereto. The reed carried contact is engageable with a stationary contact carried by the base and also with the temperature differential adjusting screw. In order to obtain different stresses in the marginal legs so as to establish different thermal characteristics thereof, a hinge spring having legs of different length is attached to the reed by any suitable means, such as by welding. The calibrating screw engages the reed beneath the hinge spring.

By having the marginal legs of different effective lengths, the temperature versus force cycle of the thermostat is such that a positive contact pressure will be maintained until the reed snaps from one position to the other. The snap action movement of the reed is independent of the rate of temperature change since the reed is unstable due to the different thermal characteristics of the marginal legs.

- The scope of this invention comprehends other means for obtaining a predetermined force relationship between two compression legs of a three-legged bimetallic ele- 2,882,370 Patented Apr. 14, 1959 ment than disclosed in the single embodiment of this invention. Thus, the compression legs could be of the same length and different widths; the same length and different thicknesses; or of different lengths by reducing the thickness of one leg while maintaining equal lengths of the hinge spring legs.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing, wherein a preferred embodiment of the present invention is clearly shown.

In the drawing:

Figure 1 is a top plan view of a thermostat constructed according to this invention.

Figure 2 is a view, partly in section and partly in elevation, taken along line 22 of Figure 1.

Figure 3 is a graph depicting the force versus deflection curve of the longer compression leg of the thermostat.

Figure 4 is a graph similar to Figure 3 showing the force versus deflection curve for the shorter compression leg of the thermostat.

Figure 5 is a composite graph on an enlarged scale depicting the operating characteristics of the metallic thermostat of this invention within the contact gap.

With particular reference to Figures 1 and 2, a thermostat is shown comprising a supporting structure 10 composed of insulating material and having a pair of threaded openings which receive screws 11 and 12. A stationary contact 13 is carried by a bracket 14 suitably attached to the supporting structure 10. A bimetallic snap acting element 15 has one end attached to the supporting structure by means of rivets 16 and carries a contact 17 which is engageable with the stationary contact 13.

The bimetallic element 15 has a pair of longitudinally extending closed slots 18 and 19 therein so as to form a three-legged strip comprising marginal or exterior legs 20 and 21 and a central leg 22. The central leg 22 is crimped as indicated in Figures 1 and 2 so as to place this leg in tension While placing the marginal legs 20 and 21 under compression. A hinge spring 23 having legs 24 and 25 is welded to the bimetallic strip 15. The hinge spring leg 24 is of greater length than the leg 25 thereby rendering the effective length of the marginal bimetal leg 20 shorter than the marginal bimetal leg 21. By having the legs 20 and 21 of different effective lengths, the thermal characteristics of these legs differ since the legs are stressed to different degrees.

The calibrating screw 11 engages the bimetallic strip 15 beneath the base portion of the hinge spring 23. The temperature differential adjusting screw 12 limits movement of the bimetallic element in the clockwise direction about the calibrating screw 11 as shown in Figure 2, and the position of the screw 12 determines the contact gap indicated by numeral 30 in Figure 2.

With particular reference to Figure 3, the force versus deflection curve for the longer leg 20 of the prestressed bimetallic element 15 is shown. In Figure 3, curve 31 indicates the force versus deflection curve at a temperature of 74 F. and numeral 32 indicates the force versus deflection curve at a temperature of 72 F. By reason of the movement of the free end of the bimetallic element 15 being limited by differential adjusting screw 12 and the stationary contact 13, the working range of leg 21 is determined by the contact gap 30 which is indicated on Figure 3. As the temperature drops from a value as indicated by point 33 at 74 to 72, the contact pressure reduced to zero, as indicated by point 34. A conventional prestressed bimetallic reed will snap from one position to the other when the contact pressure is zero since the reed is unstable, and will have a maximum contact pressure in the reverse direction as indicated by point 35. As the temperature increases, the

3 contact pressure again decreases to zero as indicated by point 36 at which point the reed is unstable and snaps in the other direction with a maximum force as indicated by point 33.

With reference to Figure 4, the temperature versus deflection characteristics of the shorter compression leg 20 are shown. Since the leg 26 has a shorter effective length than leg 21 for any given calibration of the thermostat, the thermal characteristics, and hence the force relationships in the leg 20 are different from that in the leg 21. Numeral 41 indicates the force versus deflection curve for leg 20 at 76 F. and numeral 42 indicates the force versus deflection curve at a temperature of 70 F. As the temperature of leg 21 decreases from 76 to 70 F., the contact pressure decreases from a maximum as indicated by point 43 to zero as indicated by point 44. In a conventional prestressed bimetallic reed, the leg 2% would snap to a position on the opposite side of the contact gap which is indicated by numeral 39 with a maximum contact force as indicated by point 45. Similarly upon a temperature rise the contact pressure will be reduced to Zero as indicated by point 46 at which point the leg 20 would snap to the other side of the contact gap with maximum force as indicated by point 43.

With particular reference to Figure 5, in the bimetallic reed constructed according to this invention, a predetermined force relationship exists between the legs 20 and 21. Figure depicts a graph wherein the thermal characteristics of both legs and 21 are plotted in composite form within the contact gap 30. With a calibration setting of 72 F., and an actual temperature of 74 F., the leg 21 will exert a contact pressure as indicated by point 33. At this temperature, the leg 20 will exert a contact pressure as indicated by point 48 (which coincides with point 33 in Figure 5). The total contact pressure will be the algebraic sum of that exerted by legs 20 and 21 as indicated by point 49. As the temperature drops from 74 F. to 72 F. the contact pressure exerted by leg 21 will drop to zero. However, the contact pressure exerted by leg 29 is indicated by numeral 50. Thus, the contacts are maintained in engagement until the force reversal in leg 21 exceeds the increment of force between point 50 and zero at which time the reed will snap to the opposite side of the contact gap with a force as indicated by point 51. The pressure at point 51 is the algebraic sum of the pressure exerted by legs 20 and 21.

Similarly upon a temperature rise, the contact pressure will never be reduced to zero since the reed will snap to the other side of the contact gap only when the pressure exerted by leg 20 in the reverse direction exceeds the pressure exerted by leg 21 for that temperature. For example, if the temperature rises to 74 F, the reed Will snap over and have a contact pressure as indicated by point 42. Thus, the operation of the bimetallic element is independent of the rate of temperature change since the bimetallic element is made unstable due to the diflerent thermal characteristics of the compression legs resulting from their different effective lengths. Moreover, there will always be sufficient contact pressure to maintain a low resistance connection between the contacts.

From the foregoing it is apparent that the present invention provides an inexpensive way to vary the thermal characteristics of the compression legs of a prestressed bimetallic element so as to always maintain a predetermined positive contact pressure until the bimetallic element snaps to the opposite side of the contact gap. This is accomplished by obtaining a predetermined force relationship between the two compression legs of a three-legged, snap acting bimetallic element.

While the embodiment of the invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted, as may come within the scope of the claims which follow.

What is claimed is as follows:

1. A snap acting thermostat including, a supporting structure, a snap acting bimetallic element having one end attached to said supporting structure and a contact at its other end, said bimetallic element having a pair of longitudinally extending closed slots therein forming a pair of external legs and a central leg, the central leg being crimped to place it in tension and the exterior legs in compression, said exterior legs having different thermal characteristics, and a stationary contact engageable with the contact carried by the bimetallic element whereby said contacts are always maintained in engagement with a predetermined positive pressure.

2. A thermostatic device including, a supporting structure, and a bimetallic element having one end attached to said structure and a pair of longitudinally extending slots therein forming a central leg and a pair of exterior legs, said exterior legs being stressed to different degrees.

3. A thermostatic device including, a supporting structure, a bimetallic element attached to said structure at one end and having a pair of compression legs and a tension leg, and means cooperating with said compression legs for obtaining different degrees of compression therein.

4. A thermostatic device including, a supporting structure, a bimetallic element attached to said supporting structure at one end and having a pair of compression legs and a tension leg, and means attached to said bimetallic element for rendering the effective length of one of said compression legs longer than the other compression leg.

5. A thermostatic device including, a supporting structure, and a bimetallic element attached to said supporting structure at one end and having a pair of compression legs and a tension leg, said compression legs having different thermal characteristics.

6. A thermostatic device including, a supporting structure, and a bimetallic element attached to said supporting structure at one end and having a pair of marginal compression legs and a central tension leg, said marginal compression legs having a predetermined differential force relationship.

7. A thermostatic device including, a supporting structure, a bimetallic element attached to said supporting structure having a pair of compressive legs and a tension leg, and a hinge spring attached to said bimetallic element having leg portions of different length engaging the compression legs of said bimetallic element whereby said compression legs have different elfective lengths.

8. A snap acting thermostatic switch including, a supporting structure, a bimetallic element attached to said structure at one end having a pair of compression legs and a tension leg, a contact carried by the other end of said bimetallic element, a stationary contact carried by said supporting structure, adjustable means carried by said supporting structure and cooperable with the other end of said bimetallic element for determining the gap between said contacts, and means attached to said bimetallic element having leg portions of different lengths engaging said compression legs whereby said compression legs have different thermal characteristics so as to maintain a positive pressure between said contacts until said bimetallic element snaps from a contact engaged position to a contact disengaged position.

9. A thermostatic device including, a supporting structure, a snap acting bimetallic strip having one end attached to said structure and a pair of marginal legs, and a hinge spring attached to said bimetallic strip having leg portions of different lengths engaging the marginal legs of said strip whereby said marginal legs have difierent effective lengths.

10. An article of manufacture comprising, a snap act- 5 ing bimetallic strip having a pairvof marginal legs, and a hinge spring attached to said strip having leg portions of different length engaging said marginal legs whereby said marginal legs have different effective lengths.

11. An article of manufacture comprising, a bimetallic 5 having leg portions of different length engaging said compression legs whereby said compression legs have different effective lengths.

References Cited in the file of this patent UNITED STATES PATENTS 2,349,612 Campbell May 23, 1944 2,363,280 Arnold Nov. 21, 1944 2,432,488 Peacock et al. Dec. 9, 1947 2,615,108 Lee Oct. 21, 1952 2,816,195 Holmes Dec. 10, 1957 

